Reduction of chlorates in caustic soda



April 1968 w. c. BRADBURY ETAL 3,380,806

REDUCTION OF CHLORATES IN CAUSTIC SODA Filed July 2, 1965 v JET SEPARATOR 15 HEATER A6 21 ,MOLTEN Nu 0H VAPOR DISCHARGE LINE PRODUCT 7 HOLDlN T0 FLAKTNG VESSELG OPERATION INVENTORS WALTER C. BRADBURY ROBERT D. COOPER BY 5M4, Qrw

ATTORNEYS United States Patent Ofifice 3,380,806 Patented Apr. 30, 1968 3,380,806 REDUCTION OF CHLORATES IN CAUSTIC SODA Walter C. Bradbury, Wichita, and Robert D. Cooper,

Goddard, Kans., assignors to Frontier Chemical Company, Division of Vulcan Materials Company, Wichita,

Kans., a corporation of New Jersey Filed July 2, 1965, Ser. No. 469,250 8 Claims. (Cl. 23-184) ABSTRACT OF THE DISCLOSURE The chlorate content of caustic alkali solutions is reduced by the addition of sorbitol or glycerin and heating until the chlorate present is substantially eliminated.

This invention relates to an improvement in the reduction of chlorates in caustic alkali. More particularly it relates to a process for making caustic soda or caustic potash from electrolytic salt liquor by evaporation of the water and rapid reduction of the chlorates present in the liquor by means of certain added hydroxyl compounds. Still more particularly, it relates to the reduction of chlorates in caustic alkali by sorbitol or glycerol, which are liquid and relatively non-volatile at temperatures below 150 C.

In the production of anhydrous caustic alkali such as caustic soda from electrolytic salt liquor, the alkali chlorate which is normally present therein as an impurity must be completely removed from it prior to its dehydration at elevated temperatures. Otherwise, nickel evaporators and other equipment used in the process become rapidly corroded.

In the production of caustic alkali in electrolytic liquors, it is customary to purity and concentrate the salt liquors in stages. Alkali metal chloride is usually first separated from the liquor by crystallization in a first evaporation stage, at a temperature below 150 C., whereupon the de-salted liquor is further concentrated until completely dehydrated. For instance, 50% caustic soda from the desalting evaporators is commonly evaporated further under vacuum up to a strength of about 70% or 73% NaOH using steam as a heat source and the 70% plus solution is then fed to direct fired nickel pots where the solution is substantially completely dehydrated. However, one of the problems which attends this type of operation is the corrosion of nickel or other metal equipment by the chlorate which is normally present in the caustic solution, such corrosion becoming increasingly severe as the concentration, and hence the evaporation temperature, of the liquor being evaporated is increased. Such corrosion becomes particularly troublesome when solutions containing more than 7 caustic alkali are dehydrated.

Various expedients have therefore been previously proposed for removing chlorate from caustic alkali, e.g., by addition of an aqueous sugar solution to the caustic soda solution whereby sodium chlorate present in the caustic solution is reduced at relatively moderate conditions and sodium chloride as well as Water and oxides of carbon are formed. Any carbon dioxide thus formed is, of course, converted to sodium carbonate by the caustic soda and remains in the finished product as an impurity. U.S. Patent No. 2,610,105 is representative of this prior practice. Thus, while the major corrosion problem and attendant metal contamination of the product is minimized, carbonate concentration is undesirably but unavoidably increased by this practice.

It is an object of this invention to provide an improved process for removing chlorates from aqueous caustic alkali metal hydroxide solutions. A more specific object is to provide an improved process for the substantially complete dehydration of chlorate-containing caustic soda solutions with a minimum of equipment corrosion. A still further object is to provide such a process wherein such chlorate reduction is accomplished with a minimum of product contamination. Still another object is to provide a process for reducing chlorates in caustic soda solutions by means of an agent which does not require the introduction of extraneous water into the process and which permits preheating the caustic liquor to a temperature as high as 200 C. and higher after addition of the reducing agent and before introduction of the liquor into the evaporator, thereby minimizing the heat load on the evaporator.

In accordance with the present invention, it has now been discovered that the stated objects can be met and an improvement in the production of dehydrated caustic alkali can be otbained by adding glycerin or sorbitol, the most preferably the latter to the caustic alkali liquors to be dehydrated, and heating the mixture at a temperature above about 100 C. until the chlorate present is substantially completely eliminated. The invention is particularly useful in the evaporation of concentrated caustic alkali metal hydroxide solutions, e.g., aqueous caustic soda or caustic potash solutions which contain 50% to 75% or more of the alkali metal hydroxide, e.g., to 73% NaOH and from 500 to 5,000 or more parts per million of chlorate based on the solids content of the solution, as well as incidental amounts of other compounds, e.g., up to 5%, but preferably less than 1.5% NaCl, up to 1%, but preferably less than 0.3% Na CO etc.

The stoichiometric proportions of glycerin and sorbitol required to reduce the objectionable chlorate impurity present in the caustic alkali solution can be determined from the following equations:

For sorbitol:

C H O +4NaClO 4NaCl+CO+5CO +7H 0 For glycerin:

Without wishing to imply that these equations neces sarily represent the exact chemical reactions involved, they show that approximately 0.43 part by weight of either sorbitol or glycerin is theoretically required for the reduction of one part of sodium chlorate. In practice, however, it has been found necessary to add about 3 to 6 times the indicated stoichiometric amount, i.e., about 1 to about 3 parts, preferably about 1.2 to 2.6 parts of sorbitol or glycerin per part by weight of sodium chlorate in order to obtain substantially complete elimination of the chlorate and to obtain this result within a reasonably short time. Due to the action of the hot caustic soda, some decomposion of the organic additive apparently occurs simultaneously with its reaction with sodium chlorate.

To demonstrate the unusual superiority of the present invention, tests have been made comparing sucrose, sorbitol, glycerin, other polyols, lactic acid and urea under identical conditions as reducing agents for chlorates in concentrated caustic soda solutions while heating the solutions at an elevated temperature. These tests were conducted at 200 C. with 2,000 gram samples of 73% caustic soda solution containing from about 500 to 700 ppm.

3 NaClO 1.45 percent NaCl and 0.25 percent Na CO The results obtained are summarized in Table I below.

. 4 As can be seen from these tabulated data, when using three times the stoichiometric quantity of reducing agent TABLE I.REDUCIION OF CHLORATE Run NnClO; Multiple of Minutes Percent Percent No. Treating Agent in Feed, Stoichiometric Treat NaClOa N32C03 p.p.m. Requirement Time Destroyed Increase 513 3X 15 17. 513 3X 30 32. 00 513 3X 60 65 513 3X 90 85 513 3X 120 100. 00 535 6X 46. 50 535 0X 100. 00 550 3X 15 17. 00 550 3X 30 53. 550 3X 86. 50 550 3X 90 100. 00 576 6X 15 69. 0O 57 6 6X 30 100. 00 625 3X 15 24. 50 625 3X 30 33. 50 625 3X 60 38. 00 625 3X 90 45. 50 625 3X 120 59. 50 680 6X 15 62. 00 6 .do 680 6X 30 100. 00 7 Pentaerythrltol 528 3X 30 0. 00 8 do 577 6X 30 0. O0 9-- Mannitol- 430 6X 15 3. 00 9.- do 430 6X 30 23.00 9-- do 430 6X 45 63.00 9-- do- 430 6X 60 89.5 10- Ethylene Glycol- 534 6X 30 0. 00 0. 00 11 Diethylene Glycol 521 6 X 30 0.00 0. 00 12 Triethylene Glycol 564 6 X 30 0. 00 0. 00 13 Tripropylene GlycoL 522 6X 30 0. 00 0. 00 14 Trimethylol Propane 562 6X 30 0. 00 0. 00 l5 Lactic Acid- 569 6X 15 0.00 0. 00 Urea 577 6X 15 0. 00 0. 00

The tests were conducted by adding the indicated amount and kind of additive to the respective samples of the aqueous caustic soda, heating the samples in a nickel container at 200 C. for different periods and determining the sodium chlorate and sodium carbonate contents of each sample at the end of the test. The container used in these tests was a nickel pot of 1,800 milliliters capacity which was placed in a heated, thermostatically controlled oil bath. The pot was provided with a loosely-fitting nickel lid which had two openings, one to accommodate the shaft of a motor-driven stirrer which extended down to the lower part of the container and the other to accommodate an inlet tube for passing a slow stream of nitrogen gas into the container so as to prevent any incidental oxidation of the sample by the air. Of course, in commercial operation such nitrogen blanketing is unnecessary since reduction and evaporation step is operated under vacuum.

The sodium chlorate was determined in the caustic solutions in the following manner.

Weigh a 3.0000 gram sample into a 500 cc. Erlenmeyer flask and dilute to 150 cc. with distilled water. Add from a pipet or buret an excess of 0.01 N sodium arsenite. Make the solution just acid to litmus paper with 1-3 sulfuric acid, and add 3 ml. of acid in excess, followed by 5 drops of 0.01 M osmium tetraoxide solution. Dilute to 300 ml. with distilled water and warm to 60 C. Add 2 drops of 0.025 M orthophenanthroline ferrous sulfate indicator, and titrate the excess of sodium arsenite with 0.01 N ceric nitrate at a temperature between 40 and 60 C. to a colorless or faint blue end point.

Run a blank using the same volume of 0.01 N sodium arsenite, as used in the determination diluted to 300 ml. with distilled water to which is added 3 ml. of 13 sulfuric acid, 2 drops of 0.01 M osmium tetraoxide, and 2 drops of the ortho-phenanthroline ferrous sulfate indi- (AB) X0.01391 weight of sample Percent ClO 1.27'6=percent NaClO percent 010;,

required, glycerin completely reduced sodium chlorate in the caustic soda solution after 90 minutes; sorbitol reduced percent of the chlorate after minutes and completely reduced the chlorate after minutes. In contrast, sucrose reduced only 45 percent of the chlorate after 90 minutes and only 60 percent of the chlorate after 120 minutes.

Using six times the stoichiometric quantity, sorbitol, glycerin and sucrose each produced complete reduction of the chlorates in 30 minutes but in the case oct sucrose the increase of sodium carbonate in the final product was about one and one-half times greater than in the case of glycerin and about two and one-half times greater than in the case of sorbitol. Mannitol used at six times the stoichiomctric requirement gave some chlorate reduction but after 30 minutes such reduction was less than 25% complete and even after 60 minutes such reduction still was incomplete. All other additives tried were completely unsuccessful.

In addition to causing less carbonate contamination of the product than sucrose, glycerin and sorbitol have the advantage over sucrose in that at process temperatures they are readily metered to the process as pure compounds or as concentrates inliquid or molten form. For instance, sorbitol can be conveniently handled through a proportioning group as an 80 percent concentrate, in which it is commonly available in commerce. Or the reducing agent can be added in the form of an aqueous solution containing at least 50% NaOH and at least 10% reducing agent. In contrast, the prior art taught metering the required sucrose to the process as a relatively dilute aqueous solution and, consequently, caused an added load on the evaporation system of the process. Since the normal operating temperature in evaporating 73% caustic soda is approximately 200 C. and the system is operated at reduced pressure, sorbitol, being a higher boiling polyol than glycerin, provides a greater degree of safety. Lower boiling polyols, such as ethylene glycol and diethylene glycol, have proved totally unsuited because such polyols are quickly lost by evaporation at the high processing temperature.

Furthermore, the use of either sorbitol or glycerin as the additive has the advantage over sucrose in that the slower decomposition rate of sorbitol and glycerin permits preheating the additive-containing caustic feed stream to a higher temperature, e.g., to just beneath its boiling point of about 200 C. in the case of 73% caustic soda, thereby reducing the heat load on the vacuum evaporator. Thus, when evaporating under otherwise conventional conditions, the use of sorbitol or glycerin in place of sucrose increases the evaporator capacity of a given system. When sucrose-containing caustic soda solutions are preheated to a temperature approaching 200 C., copious gassing results from the decomposition of sucrose which occurs, and the capacity of the vacuum system becomes insufficient to handle the quantity of noncondensible gas produced. In fact, it has been found possible to operate the vacuum system of the process at 1 inch Hg better vacuum when using sorbitol than when using sucrose, all other conditions being the same.

A preferred mode of this invention for concentrating caustic soda solutions while removing chlorate therefrom to minimize corrosion is illustrated by the flow diagram shown in the enclosed drawing.

Referring to this drawing, an approximately 73% aqueous caustic soda stream containing about 500 to 1,000 p.p.m. NaClO about 1 to 1.5 percent NaCl and about 0.1 to 0.3 percent Na CO is pumped from storage tank 1 by pump 3 through line 2 to a steam preheater 6 where the solution is heated at not less than substantially atmospheric pressure to a temperature below its boiling point and above 175 C., e.g., to about 185 C. Prior to the preheater an approximately 80% aqueous sorbitol solution is pumped from additive tank 4 by pump 5 and mixed with the 73% caustic feed stream in a proportion calculated to provide about 6 times the stoichiometric quantity of sorbitol required to reduce the chloride present in the soda feed stream. The efiluent from the steam preheater 6 passes to flash tank 7 wherein it is maintained at approximately 185 C. From the atmospheric flask tank 7 the treated stream is fed by pump 8 through line 9 to a vacuum evaporator 10. The evaporator is heated by a high temperature stable heat transfer liquid such as eutectic mixture of diphenyl and diphenyl oxide which is passed through line 11 to the top of the evaporator jacket and returned through line 13 to a heater 12. The liquid in evaporator 10 is evaporated at about 400 C. under reduced pressure, e.g., 24 inches Hg at the top, the water being substantially completely evaporated and the molten caustic and water vapor discharged through exit pipe 14 to separator 15 where the molten caustic passes through line 16 to a holding vessel 17 and then to a flaking or drumming operation. The water vapor passes to a barometric jet condenser 20 which supplies vacuum to evaporator 10, and the condensate is discharged through the barometric leg 21.

In the absence of indications to the contrary, all amounts, percentages and proportions of materials are expressed herein on a weight basis.

The invention described in the foregoing specification is particularly pointed out and distinctly claimed as in the appended claims.

What is claimed is:

1. In a process for reducing the chlorate content of an aqueous caustic alkali solution which contains chlorate as an impurity, the improvement which comprises adding to said chlorate-containing solution about 1 to 3 parts by weight of a reducing agent selected from the group corisisting of sorbitol and glycerin per part by weight of chlorate and heating the resulting mixture at a tempera ture above about 100 C. until said chlorate is substantilaly completely eliminated.

2. A process according to claim 1 wherein said aqueous caustic alkali solution is a solution which contains about 50 to by weight of sodium hydroxide and about 0.05 to 0.5 part by weight of sodium chlorate per parts by weight of sodium hydroxide.

3. A process according to claim 1 wherein said aqueous caustic alkali solution is a solution which contains about 50 to 75 by weight of potassium hydroxide and about 0.05 to 0.5 part by weight of potassium chlorate per 100 parts by weight of potassium hydroxide.

4. In a process for making anhydrous caustic soda wherein a relatively dilute aqueous caustic soda solution which contains chlorate as an impurity is concentrated to at least 50% NaOH by evaporation at a temperature below C. in a first evaporation stage and then further dehydrated in a subsequent stage at a more elevated temperature, the improvement which comprises adding to the concentrated solution from said first stage about 1.2 to 2.6 parts by weight of a reducing agent selected from the group consisting of sorbitol and glycerin per part by weight of chlorate, heating the resulting mixture at not less than substantially atmospheric pressure to a temperature below its boiling point and above C. in a preheating zone, thereafter maintaining the preheated solution in an evaporating zone at reduced pressure and at a temperature above the melting point of caustic soda until substantially anhydrous molten caustic soda is produced, and separating the molten caustic soda from the aqueous vapors.

5. A process according to claim 4 wherein the concentrated caustic solution from said first evaporation stage to which the reducing agent is added is a de-salted electrolytic caustic soda solution containing about 70 to 75 NaOH and about 0.05 to 0.5 part NaClO per 100 parts NaOH.

6. A process according to claim 5 wherein the reducing agent is sorbitol.

7. A process according to claim 5 wherein the reducing agent is glycerin.

8. A process according to claim 5 wherein an aqueous solution containing at least 50% NaOH and at least 10% reducing agent is added to said chlorate-containing concentrated caustic soda solution from said first evaporation stage to provide therein an amount of reducing agent equal to from 3 to 6 times the stoichiometric amount required to reduce the chlorate to chloride.

References Cited UNITED STATES PATENTS 2,610,105 9/1952 Pye 23-484 FOREIGN PATENTS 778,226 7/1957 Great Britain.

OTHER REFERENCES Badger et al.: Chemical Engineering, vol. 61, February 1954, pp. 18387.

EWARD J. MEROS. Primary Examiner. 

